The present invention relates to an optical coupling between a semiconductor laser and an optical fiber and, particularly, to a coupling structure for directly coupling a semiconductor laser and an optical fiber without using condenser means such as lens, etc., and a coupling method therefor.
One of methods for optically coupling an output light of a semiconductor laser to an optical fiber is to use an optical condenser means such as lens, etc., for condensing an output light of the laser and coupling it to the optical fiber. The other method is to place an end face of an optical fiber in close proximity to an output end face of a semiconductor laser to couple an output light of the laser directly to the optical fiber.
In either method, in order to reduce a coupling loss, the end face of the optical fiber is mirror-polished. This is because the coupling loss is considered as being increased by scattering if there is minute irregularity in the end face of the optical fiber (see, for example, Japanese Patent Application Laid-open No. S61-221705).
Since the direct coupling method mentioned above does not require any part such as lens and thus miniaturization and cost reduction are possible, realization of the latter method has been eagerly waited for these years. In the method for directly coupling a semiconductor laser to an optical fiber, however, a return light caused by Fresnel reflection at a mirror-polished end face of the optical fiber may be re-combined in an oscillator portion of the semiconductor laser, causing an oscillating state of the semiconductor laser to be unstable.
In general, in order to reduce an influence of reflection light from an end face of an optical fiber, the optical fiber and a semiconductor laser are arranged such that an optical axis of oscillation light from the semiconductor laser is not perpendicular to the end face of the optical fiber. This method is realized by tilting the optical fiber itself with respect to the semiconductor laser or polishing the end face of the optical fiber obliquely (see, for example, Japanese Patent Application Laid-open No. S58-132709). Another method for reducing the influence of reflection light is to provide an anti-reflection coating on the end face of the optical fiber (see, for example, Japanese Patent Application Laid-open No. H2-93414).
In the method in which the optical axis of the semiconductor laser is made oblique with respect to that of the optical fiber by polishing the end face of the optical fiber obliquely, it is difficult to precisely polish the end face at a predetermined angle. Particularly, in the method of directly coupling the semiconductor laser to the optical fiber, since the end face of the optical fiber must be polished in its filament state, the producibility thereof is low. In the method in which the optical fiber is tilted with respect to the semiconductor laser, a setting of an angle of tilting is difficult and further coupling loss is increased when the optical fiber and the laser are coupled.
On the other hand, in the method which uses the anti-reflection coating on the end face of the optical fiber, its producibility is very low since such coating is provided by attaching an anti-reflection film on the end face or vapor-depositing it thereon. In any of the conventional methods mentioned above, it is difficult to effectively remove the influence of reflection light from the end face of the optical fiber. For this reason, it is impossible, in the direct coupling method, to make the end face of the optical fiber sufficiently close to the oscillation face of the semiconductor laser. Since the coupling loss increases with increase of a distance between the end face of the optical fiber and the oscillation face of the semiconductor laser, there is another problem of coupling loss.